Toxoplasmic encephalitis (TE) is a life-threatening infection of the brain in AIDS patients caused by the opportunistic pathogen, Toxoplasma gondii. Drugs are available to treat acute T. gondii infection in these patients, and to suppress its re-emergence in those who are chronically infected, but for many patients the adverse effects of the drugs are severe, resulting in their discontinuation. Thus, there is a need to develop new, better-tolerated drugs to treat AIDS-related TE. This, in turn, requires a better understanding of the biology of T. gondii and the mechanisms underlying its virulence, so that critical points of vulnerability in the parasite?s life cycle can be identified and targeted. The life cycle stage of T. gondii responsible for disease pathogenesis, the tachyzoite, is highly motile. Tachyzoite motility is required for host cell invasion, migration across biological barriers, and dissemination through host tissues. T. gondii MyosinA (TgMyoA) is an unconventional myosin motor protein that plays a central role in parasite motility, and tachyzoites lacking TgMyoA are completely avirulent. The overarching goals of this project are to advance our mechanistic understanding of tachyzoite motility and to test whether small molecules targeting the motility machinery can ameliorate disease in an animal model of infection. The Specific Aims are to: (1) Determine how altering specific aspects of TgMyoA motor function affects parasite motility, by characterizing how recently identified small-molecule inhibitors of the TgMyoA motor affect its biomechanical activity and connecting these changes in motor function to effects on parasite 3D motility; and (2) Determine how inhibiting TgMyoA impacts parasite dissemination and disease progression, in vivo, to better understand the role of TgMyoA and parasite motility during infection and to provide the first direct evaluation of the TgMyoA motor as a drug target for preventing or treating toxoplasmosis. Recent technological advances have created an unprecedented opportunity to manipulate and study parasite motility in a truly integrated way. This project will capitalize on that opportunity across the full range of scales ? from the biochemical and biophysical properties of the TgMyoA motor, to the characteristics of parasite motility within a model 3D extracellular matrix, to the ability of parasites to disseminate and cause disease in infected animals. The results will therefore significantly enhance our mechanistic understanding of how T. gondii moves within its hosts to cause disease. Because TgMyoA is both essential for virulence and distinctly different from human myosins it is also a potential target for drug development; by directly testing the druggability of TgMyoA in an animal model of infection, this work will contribute to ongoing efforts to develop new and improved chemotherapeutics for managing T. gondii infections in AIDS patients.